1. Field of the Invention
The present invention relates to an image reading apparatus that converts light reflected from an original into analog image signals by a photoelectric conversion element, that converts the obtained analog image signals, to which analog signal processing is applied, into digital image signals by an analog-digital converter, and that outputs the digital image signals, and an image forming apparatus including the image reading apparatus.
2. Description of the Related Art
In recent years, the speed of reading image data has been increased, and the frequency of a reading operation clock is also increased accordingly. Consequently, it is essential to take a countermeasure against electromagnetic interference (EMI) relative to the regulations on electromagnetic wave intensity (such as Federal Communications Commission (FCC) and Voluntary Control Council for Interference by Information Technology Equipment (VCCI)).
In general, a Spread Spectrum Clock Generator (SSCG) is generally used as an effective countermeasure against the EMI. The SSCG is a method for integrally smoothing the EMI spectrum by modulating the frequency of the operation clock within a very small area at a slow cycle rate.
When the SSCG is used in an image reading apparatus, the adverse effect of modulation of the SSCG appears on an image. When the modulation of the SSCG is applied on a drive of a charge-coupled device (CCD) used as a photoelectric conversion element and an operation clock of an analog-digital (A/D) convertor, the offset level of an image varies in synchronization with the modulation cycle of the SSCG, however slight. Accordingly, a streak PP such as a horizontal streak or an orthogonal streak appears on the image (see FIG. 12).
To solve such a problem, for example, Japanese Patent Application Laid-open No. 2008-118366 discloses a technology of removing a variation component in an image signal, and preventing horizontal streaks from appearing.
In Japanese Patent Application Laid-open No. 2008-118366, an image reading apparatus includes a variation removing circuit 112 so as to function to convert incident light into analog image signals by a photoelectric conversion element, to digitalize the analog image signals by an analog-digital converter, and to output the signals. The variation removing circuit 112 drives the photoelectric conversion element 105 with a clock whose frequency is modulated, and superimposes a signal having the opposite phase but the same variation amount as those of the variation of the analog image signal onto an image signal, corresponding to a frequency change of the clock (see FIG. 13).
The variation removing circuit 112 generates a correction signal which is to be superimposed on the image signal and whose amplitude and phase are set in advance, and superimposes the correction signal on the image signal. As an example, the variation removing circuit 112 is capable of changing the bias current of a transistor connected in an emitter follower configuration.
However, in such a conventional technology, the amplitude and the phase of the correction signal to be superimposed on the image signal are set in advance. Accordingly, when the offset variations due to the effect of the SSCG fluctuate among devices, the conventional technology may fail to deal with this situation.